The Nigerian economy heavily relies on oil and gas revenue, which significantly contributes to its development. However, competition for financial performance among oil and gas companies has raised concerns among residents in the South-South region. This study examines the impact of sustainability reporting on the financial performance of six upstream oil and gas companies in South-South Nigeria, using an ex-post facto design. A total of 54 observations were drawn from nine years (2015–2023) of audited financial statements and sustainability reports sourced from company websites and the Nigerian Stock Exchange. Environmental, economic, and social reporting metrics were analyzed using content analysis based on Global Reporting Initiative (GRI) standards. Data validity was ensured through audited reports and diagnostic tests, including Variance Inflation Factor (VIF) for multicollinearity and the Breusch-Pagan test for heteroskedasticity. Descriptive statistics summarized data trends, while multiple regression analysis evaluated the relationships between sustainability reporting and financial performance, with results presented through tables, charts, and graphs. The study evaluates the impact of sustainability reporting on financial performance, revealing mixed outcomes. Environmental reporting positively but insignificantly affects ROA (β = 0.138, p = 0.063), while economic reporting has a significant negative effect (β = -0.015, p < 0.01), and social reporting shows no significant impact (β = -0.187, p = 0.596). For ROE, environmental (β = 0.0177, p = 0.8709), economic (β = 0.0314, p = 0.7596), and social (β = 0.1101, p = 0.3895) reporting display positive but insignificant effects. Economic reporting significantly influences ROCE (β = 0.179, p = 0.0051), whereas environmental (β = 0.198, p = 0.113) and social reporting (β = -0.149, p = 0.303) remain insignificant. The models, while statistically significant, exhibit weak to moderate explanatory power (R²: 0.011–0.379), indicating a limited overall impact of sustainability reporting on financial performance. Economic reporting stands out as the most influential, particularly for ROCE, highlighting the importance of strategic economic disclosures. Further research into additional determinants of financial performance is recommended to deepen understanding of these relationships.